Heart failure is a debilitating disease in which abnormal function of the heart leads in the direction of inadequate blood flow to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately eject or fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles (particularly the left ventricle) to grow in thickness in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
The current standard treatment for heart failure is typically centered on medical treatment using angiotensin converting enzyme (ACE) inhibitors, diuretics, beta-blockade, and digitalis. Cardiac resynchronization therapy (CRT) may also be employed, if a biventricular pacing device is implanted. Briefly, CRT seeks to normalize asynchronous cardiac electrical activation and resultant asynchronous contractions associated with CHF by delivering synchronized pacing stimulus to both ventricles. The stimulus is synchronized so as to improve overall cardiac function. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias. CRT and related therapies are discussed in, for example, U.S. Pat. No. 6,643,546 to Mathis, et al., entitled “Multi-Electrode Apparatus and Method for Treatment of Congestive Heart Failure”; U.S. Pat. No. 6,628,988 to Kramer, et al., entitled “Apparatus and Method for Reversal of Myocardial Remodeling with Electrical Stimulation”; and U.S. Pat. No. 6,512,952 to Stahmann, et al., entitled “Method and Apparatus for Maintaining Synchronized Pacing.”
In view of the potential severity of heart failure, it is highly desirable to detect the onset of heart failure within a patient and to track the progression thereof so that appropriate therapy can be provided. Many patients suffering heart failure already have pacemakers or ICDs implanted therein or are candidates for such devices. Accordingly, it is desirable to provide such devices with the capability to automatically detect and track heart failure.
At least some techniques have been developed for detecting heart failure and controlling responsive therapy that exploit electrical immittance signals (i.e. impedance or admittance signals) detected within the patient. See, e.g., U.S. patent application Ser. No. 11/558,194, of Panescu et al., filed Nov. 9, 2006, entitled “Closed-Loop Adaptive Adjustment of Pacing Therapy based on Cardiogenic Impedance Signals Detected by an Implantable Medical Device”; and U.S. patent application Ser. No. 12/127,963, of Wenzel et al., filed May 28, 2008, entitled “System and Method for Estimating Electrical Conduction Delays from Immittance Values Measured using an Implantable Medical Device.” Particularly effective techniques for calibrating immittance-based techniques are set forth in: U.S. patent application Ser. No. 11/559,235, by Panescu et al., now U.S. Pat. No. 7,794,404 entitled “System and Method for Estimating Cardiac Pressure Using Parameters Derived from Impedance Signals Detected by an Implantable Medical Device” and in U.S. patent application Ser. No. 12/109,304, by Gutfinger et al., filed Apr. 24, 2008 entitled “System and Method for Calibrating Cardiac Pressure Measurements Derived from Signals Detected by an Implantable Medical Device.”
In particular, techniques are described therein that exploit immittance signals to estimate electrical or mechanical conduction delays within the heart of the patient, which are, in turn, used to detect or track heart failure. Techniques are also described therein that exploit immittance signals to directly estimate cardiac pressure values within chambers of the heart of the patient, which are, in turn, used to detect or track heart failure.
Although the techniques are effective, it would be desirable to provide alternative techniques for detecting heart failure that exploit immittance signals measured within a patient, but which do not rely on cardiac pressure estimates or conduction delay estimates. It is to this end that aspects of the present invention are primarily directed.